Polytetrafluoroethylene (PTFE) has demonstrated utility as a material in many different areas. As an industrial material, such as for example a gasket, polytetrafluoroethylene has exhibited utility as a material for use in harsh chemical environments which normally degrade many conventional metals and polymeric materials. Polytetrafluoroethylene is also usable over a broad temperature range, from as high as 260.degree. C. to as low as near -273.degree. C.
However, conventional non-porous polytetrafluoroethylene gasket materials which have been compression molded or extruded and subsequently heated to a temperature above 345.degree. C. exhibit poor mechanical properties, such as low tensile strength and low cold flow resistance, thereby limiting their use in areas requiring a measure of physical strength and resistance to cold flow (also known as creep).
Polytetrafluoroethylene may be produced in a porous expanded form as taught in U.S. Pat. No. 3,953,566 to Gore. Porous expanded polytetrafluoroethylene (ePTFE) is of a higher strength than unexpanded forms of polytetrafluoroethylene, has the chemical inertness of polytetrafluoroethylene, and when used as a gasket has even a wider temperature range of use, to as high as 315.degree. C. An example of a porous expanded polytetrafluoroethylene gasket material is Gore-tex.RTM. Joint Sealant (manufactured by W. L. Gore & Associates, Inc, Elkton, Md.). Porous expanded polytetrafluoroethylene joint sealants provide excellent seals in applications having relatively wide sealing surfaces and where sufficient clamping loads can be applied to them so that, as the sealant spreads between the sealing surfaces to form a thin wide gasket, the level of compressive stress needed to densify the gasket and to provide the desired sealability is developed. Consequently, they are not well suited to applications having narrow sealing surfaces or requiring relatively thick gaskets.
Porous expanded polytetrafluoroethylene gasket materials which have not been fully compressed will not perform as a barrier to organic liquids having surface tensions under 50 dynes/cm.sup.2. Also, they will exhibit a higher measure of creep (cold-flow) which results in a loss of thickness and an increased width of the gasket over time. Both the loss of gasket thickness and the increase in the gasket width contribute to a lower surface stress of the gasket in service.
Polytetrafluoroethylene sealant materials consisting of polytetrafluoroethylene cores upon which are wrapped tapes of polytetrafluoroethylene are known in the art. For example, Gore-tex.RTM. Valve Stem Packing (manufactured by W. L. Gore & Associates, Inc., Elkton, Md.) and Inertex.RTM. Valve Stem Packing (available from Inertech, Inc., Monterey Park, Calif.), consist of porous expanded polytetrafluoroethylene cores upon which are wrapped tapes of porous expanded polytetrafluoroethylene. Both of these commercially available wrapped PTFE sealant materials are suitable for use as a compression packing where they are confined within a defined volume, however, when used as a gasket in an unconfined volume they exhibit undesirable creep characteristics over a period of time when under a compressive load. U.S. Pat. No. 5,160,773 (to Sassa) describes a sealing material of polytetrafluoroethylene felt encapsulated by a porous polytetrafluoroethylene sheet laminated to a melt-processible thermoplastic fluoropolymer. The encapsulated polytetrafluoroethylene felt material is engineered for use as a "wiper" seal for moving surfaces in which very low clamping forces are applied to the seal surfaces to obtain a seal against low pressure fluids. The encapsulated felt material is not suited for use as a static seal against gases or fluids under pressure as a very high clamping load is required to compress it and, as with the materials described above, the encapsulated felt material deforms to a thin wide shape as it becomes fully compressed and exhibits undesirable creep characteristics. The outer wrap of the encapsulated felt is intended only to render the seal liquid-impermeable and does not have sufficient strength to retard cold-flow of the core under high compressive stresses.
As a result of dimensional changes due to cold flow or creep, many commercially available polytetrafluoroethylene gaskets require additional clamping force applied to the gasket some period of time after installation of the gasket. This is a highly undesirable property in a gasket material. Repetitive adjustment of clamping force is not possible in uses where a specific height of gasket must be maintained during use for functional or safety reasons. For example, when used as static seals between multiple plates of plate-and-frame heat exchangers and plate-and-frame filters any creep-induced thickness reduction of the gasket material between the plates is additive and, when multiplied by the number of plates in the unit, can result in a significant reduction in the gasket clamping load which can lead to severe uncorrectable leakage problems.